Coverage is an important feature for measuring a communications system. Coverage represents a maximum coverage area in which a user can obtain a service (properly receiving and transmitting data according to a specific proportion). In a communication transmission process, a signal transmitted by a transmit end experiences various fading and losses, together with interference and noise before reaching a receive end. A longer transmission distance indicates higher signal attenuation, which degrades coverage performance. Good coverage performance may help reduce base station construction costs, reduce maintenance costs, and increase access users. Therefore, coverage enhancement has always been a research focus.
Downlink synchronization is the first step for a base station to establish a communications connection to a terminal. In an existing GSM (Global system for mobile communications) system, two channels are required to perform downlink synchronization: an FCCH frequency correction channel) and an SCH (synchronization channel), where a main function of the FCCH is to perform frequency synchronization. The second step is to implement time domain synchronization by using an SCH channel, so as to complete a whole synchronization process. However, in the existing GSM system, the FCCH and the SCH can be transmitted only in a first timeslot, and an FCCH and an SCH are transmitted once at specific time intervals. Consequently, a synchronization channel of the existing GSM system can hardly meet a requirement for enhancing coverage by 20 dB.